Description Chain Home

1 description

1.1 mechanical layout
1.2 transmitter details
1.3 receiver details
1.4 distance , bearing measurement
1.5 altitude measurement
1.6 raid assessment
1.7 fruit machine

description
mechanical layout

three of 4 transmitter towers of bawdsey ch station seen in 1945. antennas proper visible @ extreme right. these towers, of chain home, built j. l. eve construction.

chain home radar installations composed of 2 sites. 1 compound contained transmitter towers associated structures, , second compound, within few hundred metres distance, contained receiver masts , receiver equipment block operators (principally waaf, women s auxiliary air force) worked. ch system was, modern terminology, bistatic radar , although modern examples have transmitters , receivers far more separated.

the transmitter antenna consisted of 4 steel towers 360 feet (110 m) tall, set out in line 180 feet (55 m) apart. 3 large platforms stationed on tower, @ 50, 200 , 350 feet off ground. 600 ohm transmission cable suspended top platform ground on either side of platform (only on inside of end towers). between these vertical feed cables antennas proper, 8 half-wave dipoles strung between vertical cables , spaced ½ of wavelength apart. fed alternating sides entire array of cables in-phase, given ½ wavelength spacing. located behind each dipole passive reflector wire, spaced 0.18 wavelength back.

the resulting curtain array antenna produced horizontally polarised signal directed forward along perpendicular line of towers. direction known line of shoot, , aimed out on water. broadcast pattern covered area of 100 degrees in fan-shaped area, smaller side lobe rear, courtesy of reflectors, , smaller ones sides. when signal reflected off ground underwent ½ wavelength phase-change, caused interfere direct signal. result series of vertically-stacked lobes 5 degrees wide 1 degree off ground vertical. system later expanded adding set of 4 additional antennas closer ground, wired in similar fashion.

the receiver consisted of adcock array consisting of 4 240 foot (73 m) tall wooden towers arranged @ corners of square. each tower had 3 sets (originally two) of receiver antennas, 1 @ 45, 95 , 215 feet off ground. mean height of transmitter stack 215 feet, why topmost antenna positioned @ same altitude in order produce reception pattern identical transmission. set of motor-driven mechanical switches allowed operator select antenna active. output of selected antenna on 4 towers sent single radiogoniometer system (not watt s own huff-duff solution). connecting antennas in x-y pairs horizontal bearing measured, while connecting upper , lower antennas allowed same goniometer used measure vertical angle.

two physical layout plans used, either east coast or west coast . west coast sites replaced steel lattice towers simpler guy-stayed masts, although retained same wooden towers reception. east coast sites had transmitter , receiver blocks protected earth mounds , blast walls, along separate reserve transmitter , receivers in small bunkers attached 120 ft aerial masts. these reserves in close proximity respective transmitter/receiver sites, in neighbouring field. west coast sites relied on site dispersal protection, duplicating entire transmitter , receiver buildings.

transmitter details

chain home transmitter, raf air defence radar museum (2007)



chain home transmitting valve, london science museum. valve capable of being dismantled , consequently had continuously vacuum pumped while operating. done via piping left.

operation began type t.3026 transmitter sending pulse of radio energy transmission antennas hut beside towers. each station had 2 t.3026 s, 1 active , 1 standby. signal filled space in front of antenna, floodlighting entire area. due transmission effects of multiple stacked antennas, signal strong directly along line of shoot, , dwindled on either side. area 50 degrees either side of line filled enough energy make detection practical.

the type t.3026 transmitter provided metropolitan-vickers, based on design used bbc transmitter @ rugby. unique feature of design demountable tubes, opened service, , had connected oil diffusion vacuum pump continual evacuation while in use. tubes able operate @ 1 of 4 selected frequencies between 20 , 55 mhz, , switched 1 in 15 seconds. produce short pulses of signal, transmitter consisted of hartley oscillators feeding pair of tetrode amplifier tubes. tetrodes switched on , off pair of mercury vapour thyratrons connected timing circuit, output of biased control , screen grids of tetrode positively while bias signal kept turned off.

stations arranged fan-shaped broadcast patterns overlapped cover gaps between stations. ensure stations did not broadcast @ same time, power national grid used provide convenient phase-locked 50 hz signal available across entire nation. each station equipped phase-shifting transformer allowed trigger @ specific point on grid waveform, selecting different point each station avoid overlap. output of transformer fed dippy oscillator produced sharp pulses @ 25 hz, phase-locked output transformer. locking soft , short-term variations in phase or frequency of grid filtered out. system of spacing transmissions out in time known running rabbits .

during times of strong ionospheric reflection, @ night, possible receiver see reflections ground after 1 reflection. address problem, system later provided second pulse repetition frequency @ 12.5 pps, meant reflection have greater 6,000 miles (9,700 km) before seen during next reception period.

receiver details

in addition triggering broadcast signal, output of transmitter trigger signal sent receiver hut. here fed input time base generator drove x-axis deflection plates of crt display. caused electron beam in tube start moving left-to-right @ instant transmission completed. due slow decay of pulse, of transmitted signal received on display. signal powerful overwhelmed reflected signal targets, meant objects closer 5 miles (8.0 km) not seen on display. reduce period point required receiver hand-tuned, selecting decoupling capacitors , impedance of power supplies.

the receiver system, built a.c. cossor tre design, multiple-stage superheterodyne. signal selected antennas on receiver towers fed through radiogoniometer , three-stage amplifier, each stage housed in metal screen box avoid interference between stages. each stage used class b amplifier arrangement of ef8s, special low noise, aligned-grid pentodes. output of initial amplifier sent intermediate frequency mixer, extracted user-selectable amount of signal, 500, 200 or 50 khz selected switch on console. first setting allowed of signal through, , used under circumstances. other settings available block out interference, did blocking of signal reduced overall sensitivity of system.

the output of mixer sent y-axis deflection plates in specially designed high-quality crt. reasons not explained in literature, arranged deflect beam downward increasing signal. when combined x-axis signal time base generator, echoes received distant objects caused display produce blips along display. measuring centre point of blip against mechanical scale along top of display, range target determined. measurement later aided addition of calibrator unit or strobe, caused additional sharp blips drawn every 10 miles (16 km) along display. markers fed same electronic signals time base, calibrated.

distance , bearing measurement

chain home display showing several target blips between 15 , 30 miles distant station. marker @ top of screen used send range fruit machine.



the operator display of ch system complex affair. large knob on left goniometer control sense button make antenna more directional.

determining location in space of given blip complex multi-step process. first operator select set of receiver antennas using motorized switch, feeding signals receiver system. antennas connected in pairs, forming 2 directional antennas, sensitive along x or y axis, y being line of shoot. operator swing gonio , or hunt , , forth until selected blip reached minimum deflection on display (or maximum, @ 90 degrees off). operator measure distance against scale, , tell plotter range , bearing of selected target. operator select different blip on display , repeat process. targets @ different altitudes, operator might have try different antennas maximize signal.

on receipt of set of polar coordinates radar operator, plotter s task convert these x , y locations on map. provided large maps of operational area printed on light paper stored future reference. rotating straightedge centrepoint @ radar s location on map fixed on top, when operator called angle plotter rotate straightedge angle, along pick off range, , plot point. range called operator line-of-sight range, or slant range, not over-ground distance station. calculate actual location on ground, altitude had measured (see below) , calculated using simple trigonometry. variety of calculators , aids used in calculation step.

as plotter worked, targets updated on time, causing series of marks, or plots, appear indicated targets direction of motion, or track. track-tellers standing around map relay information via telephone filter room @ raf bentley priory, dedicated telephone operator relayed information plotters on larger map. in way reports multiple stations re-created single overall view.

due differences in reception patterns between stations, differences in received signals different directions @ single station, reported locations varied target s real location varying amount. same target reported 2 different stations appear in different locations on filter room s plot. job of filter room recognize these same plot, , re-combine them single track. on tracks identified number, used future communications. when first reported tracks given x prefix, , h hostile or f friendly once identified. data sent down telephone network group , section headquarters plots again re-created local control on fighters.

the data went sideways other defence units such royal navy, army anti-aircraft gun sites, , raf barrage balloon operations. there comprehensive liaison civil authorities, principally air raid precautions.

altitude measurement

plotting , reporting tracks manpower intensive operation. image shows receiver station @ raf bawdsey, home of ch development. commanded flight officer wright, on phone. radar operator visible in background, right of centre. communicated plotter, in foreground wearing headphones, via intercom readings made out under attack.

due arrangement of receiver antennas, sensitive area had number of side lobes allowed reception @ multiple vertical angles. typically operator use upper set of antennas @ 215 feet (66 m), had clearest view of horizon. due half-wave interference ground, main lobe antenna directed @ 2.5 degrees above horizontal, sensitive region extending 1 3 degrees. @ ground gain zero, allowed aircraft escape detection flying @ low altitudes. second lobe extended 6 12 degrees, , on. left distinct gap in reception pattern centred @ 5.2 degrees.

this reception pattern provided ch relatively accurate way estimate altitude of target. this, motorized switch in receiver hut used disconnect 4 receiver masts , instead select 2 vertically displaced antennas on 1 mast. when connected radiogoniometer, output on display effected relative signal strength of 2 lobes, rather relative strengths in x , y in horizontal plane. operator swung radiogoniometer looking peak or minimum reception, before, , noted angle.

the number reported operator line-of-sight range target, or slant range, included components of both horizontal distance , altitude. convert real range on ground, plotter used basic trigonometry on right angle triangle; slant range hypotenuse , open angle measurement radiogoniometer. base , opposite sides calculated, revealing distance , altitude. important correction curvature of earth, became significant @ ranges ch worked at. once calculated, allowed range plotted, revealing grid square target, reported chain.

when target first detected @ long range, signal typically did not have enough of return in second lobe perform height finding. became possible aircraft approached station. problem recur target centred in second lobe, , forth. additionally, not possible determine difference between signal being compared between first , second or second , third lobe, caused ambiguity @ short ranges. however, altitude determined long before this, tended not problem in practice.

unfortunately pattern left set of distinct angles reception in both lobes low. address this, second set of receiver antennas installed @ 45 feet (14 m). when 2 lower sets of antennas used, pattern shifted upward, providing strong reception in gaps , @ cost of diminished long-range reception due higher angles.

raid assessment

another critical function of ch operators estimate number , type of aircraft in raid. gross level of overall size determined strength of return. more accurate determination made observing beat rate of composite echoes, way grew , diminished on time entered different sections of antenna reception pattern. aid this, operator reduce pulse length 6 microseconds (from 20) push-button. improved range resolution, spreading blip out on display @ cost of lower returned energy.

raid assessment largely acquired skill , continued improve operator experience. in measured tests, experimenters found acquired skill great experienced operators pick out targets returns less current signal-to-noise ratio. how accomplished great mystery @ time–the operators spotting blips in static larger signal. believed form of stochastic resonance.

fruit machine

the fruit machine simplified measurement , calculation, driving plotter directly.

operating ch station manpower-intensive situation, operator in transmitter hut, operator , assistant in receiver hut, , many 6 assistants in receiver hut operating plotters, calculators , telephone systems. in order provide 24-hour service, multiple crews needed, along number of service , support personnel. multiplied reporting hierarchy, required similar numbers of waafs @ each level of dowding system hierarchy.

plotting angle of target simple process of taking gonio reading , setting rotating straightedge value. problem determining along straightedge target lay; radar measured slant range straight-line distance target, not distance on ground. distance affected target s altitude, had determined taking time-consuming altitude measurements. additionally, altitude affected range, due curvature of earth, imperfections in local environment, caused lobes have different measurements depending on target angle.

as no small part of manpower required dedicated calculation , plotting, great reduction made using automation possible. started use of various mechanical aids; these replaced fruit machine, electromechanical analogue computer of complexity. replicated of these devices , tables in electrical form. electrical repeater, or synchro, added gonio dial. measure range, new dial added moved mechanical marker selected blip on display. when particular target selected, operator pushed button activate fruit machine, read these inputs. in addition inputs, fruit machine had series of local corrections both angle , altitude, measured calibration flights , stored in machine in telephone uniselectors. these corrections automatically added calculation, eliminating time-consuming lookup of these numbers tables. output altitude, allowed plotters determine proper over-ground distance target.

later versions of fruit machine upgraded directly output position of aircraft no manual operation. using same buttons send settings machine, operator triggered system , outputs used drive t-square-like indicator on chart, allowing operator read calculated location directly. reduced number of people needed @ station , allowed station reorganized more compact form. no longer did operator call readings out plotters; sat directly beside plotting table see if results looked right, while tellers see plot , call area plotting room. further upgrade allowed data sent local plotting room automatically on phone lines, further reducing required manpower.

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